This invention relates to a tact switch having a cup-shaped movable contact.
An example of the prior art tact switch will be described below with reference to FIG. 1. The tact switch comprises a housing 10, a top lid 20 fitted in and covering the housing 10, a cup-shaped movable contact piece 30, a movable contact actuator member 40, a central fixed contact 51 and fixed contact plates 52.
The housing 10 is molded from a synthetic resin material and comprises a generally rectangular bottom plate 11, opposed end walls 12 and opposed side walls 13, the adjacent end walls 12 and side walls 13 being integrally connected together at their ends. The bottom plate 11 has an upper flat surface on which the central fixed contact 51 and fixed contact plates 52 are integrally molded and exposed. A terminal 511 is joined with and extends continually from the central fixed contact 51 and through the interior of the bottom plate 11. Terminals 521 are joined with and extend continually from the fixed contact plates 52. Each of the opposed end walls 12 has a closure tab receiving slot 121 cut down from the top edge in the center thereof. Each of the opposed side walls 13 has a broad guide window 131 cut down from the top edge in the center thereof. Each side wall 13 has mating recesses 132 formed in the outer surface thereof adjoining the opposite sides of the broad guide window 131 and communicating with and extending horizontally from the guide window 131.
The top lid 20 closing the housing 10 is made by stamping and plastically forming a thin metal sheet and comprises a cover plate 21 which forms a main body of the lid. The cover plate 21 has a mating closure tab 22 depending from each of the opposed end edges in the middle thereof. Depending from each of the opposed side edges of the cover plate 21 adjacent each of the opposite ends is an L-shaped engagement lug 23. The cover plate 21 further has a projecting tab 24 stamped out thereof in the center and bent obliquely.
The cup-shaped movable contact piece 30 is made by plastically forming a thin metal sheet such as phosphor bronze or the like having stiff resilience and has an outer peripheral lower end face 31 lying in a common plane. Reference numeral 32 is an apex region of the cup-shaped movable contact piece 30.
In this tact switch, the cup-shaped movable contact piece 30 is placed on the upper surface of the bottom plate 11 of the housing 10 with its outer peripheral lower end face 31 in contact with the exposed surfaces of the two fixed contact plates 52 and with the convex side facing upwardly. The cup-shaped movable contact piece 30 may be constructed by appropriately selecting the material and stiff resilience of the thin metal sheet of which the contact piece 30 is formed as well as by designing the geometry thereof such that downwardly directed depressing pressure applied onto the apex region 32 will flip or invert the contact piece 30 downwardly beyond the dead center (overcenter) to deform the apex region 32 so as to project downwardly convexly and that upon the downward depressing pressure being released, the contact piece 30 will flip back upwardly beyond the dead center by its own stiff resilience to spontaneously return to its original position with the apex region 32 projecting upwardly convexly.
The movable contact actuator member 40 is molded from a synthetic resin material generally in the form of a frame structure including a generally quadrilateral frame portion 41. The frame portion 41 includes two sides opposed in the pressure actuating direction as indicated by an arrow 4A, on one side of which it has an operating portion 421 which is longer than the width of the guide window 131. Two slide portions 42 are formed integrally with and extend parallel to the pressure actuating direction inwardly from the operating portion 421 adjacent its opposite ends. Integrally formed with and protruding in the pressure actuating direction outwardly from the other of the two opposed sides in the central portion thereof is a supporting extension 43. It should be noted here that the distance between the outside surfaces of the two slide portions 42 is approximately equal to the width of the guide window 131 formed through one of the side walls 13 of the housing 10 so as to permit sliding movement of the slide portions 42 through the guide window 131. Likewise, the supporting extension 43 is slidingly movable through the guide window 131 in the other side wall 13. The operating portion 421 is formed integrally with the outer ends of the slide portions 42 so as to bridge the two portions. It is seen that the opposite ends of the operating portion 421 extend slightly beyond the respective slide portions 42 perpendicularly to the length of the slide portions 42. An actuating lever 44 is formed integrally with the operating portion 421 in the central portion thereof and extends parallel to the sliding direction inwardly from the operating portion 421 between the two slide portions 42. The actuating lever 44 has an actuating protrusion 45 formed integrally therewith.
Assembly of the tact switch is performed as follows: First, the outer peripheral lower end face 31 of the cup-shaped movable contact piece 30 is brought into abutment against the upper surface of the bottom plate 11 of the housing 10 on which the fixed contact plates 52 are exposed, with the cup 30 inverted on the bottom plate 11, that is, with the cup bottom facing upward. In this state, the cup-shaped movable contact piece 30 has its outer peripheral lower end face 31 contacted with the exposed surfaces of the two fixed contact plates 52. Next, the supporting extension 43 of the movable contact actuator member 40 is fitted into the guide window 131 in the associated side wall 13 of the housing 10 while the two slide portions 42 are likewise fitted into the guide window 131 in the other side wall 13 whereby the movable contact actuator member 40 is mounted to the housing 10.
Finally, the top lid 20 is fitted and secured to the housing 10. More specifically, the mating closure tabs 22 depending from the opposed end edges of the cover plate 21 are positioned in vertical alignment with the respective closure tab receiving slots 121 formed in the opposite end walls 12 of the housing 10 while the L-shaped engagement lugs 23 depending from the opposed side edges of the cover plate 21 adjacent the opposite ends thereof are positioned in vertical alignment with the respective mating recesses 132 formed horizontally in the opposite side wall 13 adjacent the opposite ends thereof. In this state, the cover plate 21 is pressed down to cause the horizontal legs of the L-shaped engagement lugs 23 to be snapped into the respective mating L-shaped recesses 132. The downward depressing of the cover plate 21 is effected against the repulsing force of the cup-shaped movable contact piece 30 while depressing the apex region 32 of the cup-shaped movable contact piece 30 by means of the projecting tab 24 lanced and bent out of the central portion of the cover plate 21, the actuating lever 44 and the actuating protrusion 45 formed on the lower surface of the actuating lever 44 adjacent the forward end thereof.
With the thus mated and assembled tact switch in its normal state with the slide portions 42 of the movable contact actuator member 40 withdrawn from the associated guide window 131 of the housing 10, the cup-shaped movable contact piece 30 with its apex region projecting convexly upwardly is slightly downwardly biased by means of the projecting tab 24, the actuating lever 44 and the actuating protrusion 45 while the outer peripheral lower end face 31 is in contact with the upper surface of the bottom plate 11 of the housing 10 and the exposed surfaces of the fixed contact plates 52. The cup-shaped movable contact piece 30 is not in contact with the central fixed contact 51 as its apex region 32 is projecting convexly upwardly in this condition. It is thus to be understood that in the normal condition the central fixed contact 51 and the two fixed contact plates 52 are in their disengaged (OFF) position with the forward end of the actuating lever 44 adjacent to the slant surface of the projecting tab 24.
At this point, when the operating portion 421 is operated to press the movable contact actuator member 40 into the housing 10, the forward end of the actuating lever 44 is elastically displaced downwardly as it is moved in sliding engagement with the slant surface of the projecting tab 24 whereby the actuating protrusion 45 formed on the lower surface of the actuating lever 44 adjacent the forward end thereof is brought into engagement with the cup-shaped movable contact piece 30 to depress and deform the latter downwardly. The downwardly depressed deformation of the cup-shaped movable contact piece 30 is increased as the depression of the actuating protrusion 45 progresses until the actuating protrusion 45 enters into the apex region 32 of the cup-shaped movable contact piece 30 whereupon downward deformation of the cup-shaped movable contact piece 30 exceeds the dead center and the apex region 32 is flipped over or inverted downwardly into contact with the central fixed contact 51. This contact bridges the two fixed contact plates 52 and the central fixed contact 51 with the cup-shaped movable contact piece 30 to make contact therebetween. When the depressing operation on the operating portion 421 is released, the actuator member 40 is returned to its extended position under the elastic restoring force of the actuating lever 44 and the restoring force of the cup-shaped movable contact piece 30 whereupon the cup-shaped movable contact piece 30 is flipped over back to its original position to break the bridging contact between the two fixed contact plates 52 and the central fixed contact 51.
With the prior art tact switch as described above, when the operating portion 421 is in its released position, the cup-shaped movable contact piece 30 is in the position with the cup bottom facing upward such that the outer peripheral lower end face 31 is in contact with the fixed contact plates 52 and the upper surface of the bottom plate 11 of the housing 10 on which the contact plates are disposed, whereby the air is confined between the upper surface of the bottom plate 11 of the housing 10 and the cup-shaped movable contact piece 30. This air is compressed and expelled out of the cup as the collapsing deformation of the cup-shaped movable contact piece 30 progresses until it is flipped over downwardly beyond the dead center. Then, as the movable contact piece 30 is released from the depressing operation to be allowed to flip back to its original upwardly convex condition, the air is drawn in and again confined between the interior of the cup-shaped movable contact piece 30 and the upper surface of the bottom plate 11. The cup-shaped movable contact piece 30 thus expels and draws in the air each time it is flipped over downwardly and restores to its original position. During the repeated processes of expelling and drawing the air, especially while the cup-shaped movable contact piece 30 flips over from the downwardly flipped position and returns to its original condition, the drawn air may entrain with it any dust and debris present on and in the vicinity of the surface of the bottom plate 11 into the interior of the cup-shaped movable contact piece 30 and allow it to deposit on and in the vicinity of the surface of the central fixed contact 51. Such debris deposited on and in the vicinity of the surface of the central fixed contact 51 will be lodged between the central fixed contact 51 and the apex region 32 of the cup-shaped movable contact piece 30 while the movable contact piece 30 is being inverted downwardly, resulting in poor contact between the two.